1. Field of the Invention
The present invention relates to paperboard food packages. More particularly, the invention relates to a process for heat sealing paperboard food packages and the packages produced by the process.
2. Description of the Prior Art
One or two serving portions of precooked and/or frozen food are frequently packaged for consumer distribution in paperboard trays. Such trays are folded from preprinted and die-cut bleached sulphate paperboard.
Covers for these paperboard trays may take one of several forms including a top flap that is an integral continuation of the same paperboard sheet from which the tray is erected, such top flap being crease hinged to one sidewall of the tray. Another type of lid has been an independent paperboard sheet that is adhesively secured or plastic fuse bonded to a small perimeter flange folded from the upper edge of the tray sidewalls.
To protect the food contacting internal surfaces of the paperboard tray structure from moisture penetration, the internal surfaces are coated with one or more continuous films of extruded thermoplastic. This internal moisture barrier is applied as a free-flowing curtain to the paperboard while still in the continuous web form: prior to printing and cutting. Polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) are merely three of the more popular thermoplastic polymers used for this purpose.
For the purpose of tray structure protection, the single internal polymer coating is sufficient. However, if a paperboard tray closure is required, it may be necessary to coat the tray and cover with an external coating of thermoplastic for the primary function of heat sealing the tray cover to the tray sides or flanges. This necessity arises from the material construction of bleached sulphate paperboard.
As with all other foudrinier laid paper webs, bleached paperboard is produced with a "top side" and a "wire side." For food packaging, the top side is preferred for the thermoplastic food moisture barrier. The wire side is given a clay coating that is smoothed by calendering to provide a superior print surface. This clay coated paperboard surface is a poor receptor of polymer coatings, however. Consequently, it is difficult to secure a reliable heat seal between the interior thermoplastic surface of a paperboard tray against a "naked" clay coated exterior surface.
A specific example arises with respect to a tray having an integrally hinged cover with flap extensions from the cover plane for overlapping three tray sidewalls. The tray contents are tightly enclosed by bonding the cover flap extensions against the three tray sidewalls. Production line pace permits less than 1 second to secure the flap-to-wall bond. The present invention focuses upon the mechanics of this bond.
As previously inferred, the predominately practiced prior art merely avoids the polymer-to-naked clay surface bonding problem by curtain coating the entire clay-coated wire side of the paperboard web with an extruded film of extremely hot, viscously flowing polymer: in the same manner as the food moisture barrier is applied to the paperboard web top side. Frequently, the naked clay surface of the wire side is pretreated by open flame or corona discharge immediately prior to hot polymer contact to enhance deep bonding of the polymer with the topically applied clay and underlying paper fiber. By this expedient, the underside surface of a tray cover flap and the exterior surface of the tray sidewall are coated with the same or at least compatible polymers and are easily fuse bonded at low tack temperature and joint compression pressure.
However, polymer coating an entire exterior package surface for the singular reason of effecting a reliable, rapid and low pressure heat seal with the top closure tabs produces several collateral consequences that must be accommodated. The first, obvious, consequence is economic. Proportionately, the exterior polymer coat adds significantly to the total package cost. Moreover, only a small proportion of the full surface coating is actually functional for the purpose applied. A second, adverse consequence of an exterior polymer coating is the necessity for using specialized, volatile inks for the colorful sales and informational graphics applied to the exterior surfaces of these packages. Use of these inks creates additional economic, safety and environmental concerns. The exterior polymer coat is softer than the naked clay coat and therefore mars and scratches more easily. Finally, the additional polymer coating further complicates repulping and recycling the fiber composition of the package.
As additional factors to the present invention prior art and development, it should be understood that a typical commercial food tray filling line advances at the rate of 60 to 120 units per minute. Consequently, any step or process in the continuous production line that requires a full stop of the subject unit must be accomplished in one second or less. Other processing steps are performed on a moving unit. Moreover, once the tray is filled with food product and the cover positioned, the tray inside surfaces are inaccessible for nip pressure backing. Any force applied to a cover flap for sealing against a tray side wall must be less than the crushing capacity of the erected tray. Frequently, only a gentle touch is permissible.
In addition to the previously described expedient of heat fusing polymer curtain coatings applied to both paperboard surfaces, the prior art has also relied upon both cold set and hot set adhesives to achieve a better lid-to-tray seal. Each of these adhesive sealing devices carry respective adverse consequences, however. Cold set adhesives are extremely slow setting and therefore incompatible with a production of 120 units per minute. Hot melt adhesives have relatively low softening temperatures which are incompatible with typical oven temperatures used when the food within the package is heated for consumption.
It is an object of the present invention to provide a process for sealing paperboard food packages having a polymer barrier coating applied only to the inside package surface.
Another object of the present invention is to provide paperboard food packages having only an interior surface coating of polymer for food moisture containment that is quickly and easily heat sealed to an opposite side having graphics applied to a naked clay and/or paperboard surface.
Another object of the present invention is to provide a precisely positioned spot area of polymer primer to a paperboard surface that is otherwise devoid of a plastic coating whereby a polymer coated paperboard surface may be easily heat fused thereto.
Another object of the present invention is to provide relatively small, precisely positioned, cooperative areas of polymer primer to a paperboard surface that is otherwise devoid of plastic coating whereby predetermined portions of such unplasticized paperboard surface may be quickly heat fused together.